OAK-659 Move purge logic for transient changes below the NodeBuilder interface
This initial fix still keeps transient changes from commit hooks in memory when running with a MicroKernel. See TODOs in org.apache.jackrabbit.oak.kernel.KernelNodeState.builder. When running on SegmentMK the relevant implementations in SegmentNodeStore fall back to their defaults, might not be optimal.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1513469 13f79535-47bb-0310-9956-ffa450edef68